Poly α-olefins comprise one class of hydrocarbon lubricants which has achieved importance in the lubricating oil market. These materials are typically produced by the polymerization of α-olefins in the presence of a catalyst such as AlCl3, BF3, or BF3 complexes. Exemplary α-olefins for the manufacture of PAO range from 1-octene to 1-dodecene, with 1-decene being a preferred material. Polymers of higher olefins, such as 1-tetradecene, as described in WO 99/38938 and elsewhere, and lower olefins, such as ethylene and propylene including copolymers of ethylene with higher olefins, as described in U.S. Pat. No. 4,956,122 and elsewhere, can also be used. Oligomerization is typically followed by fractionation and by a step of hydrogenation to remove unsaturated moieties in order to obtain the desired product slate. In the course of hydrogenation, the amount of unsaturation is generally reduced by greater than 90%.
PAOs are commonly categorized by the numbers denoting the approximate viscosity, in centistokes (cSt), of the PAO at 100° C. PAO products may be obtained with a wide range of viscosities varying from highly mobile fluids with a nominal viscosity of about 2 cSt at 100° C. to higher molecular weight, viscous materials which have viscosities exceeding 100 cSt at 100° C. Viscosities as used herein are Kinematic Viscosities determined at 100° C. by ASTM D-445, unless otherwise specified. The term “nominal” as used herein means that the number has been rounded to provide a single significant figure.
Processes for the production of PAO lubricants have been the subject of numerous patents, such as U.S. Pat. Nos: 3,149,178; 3,382,291; 3,742,082; 3,780,128; 4,045,507; 4,172,855; and more recently U.S. Pat. Nos. 5,693,598; 6,303,548; 6,313,077; U.S. Applications 2002/0137636; 2003/0119682; 2004/0129603; 2004/0154957;. and 2004/0154958, in addition to other patent documents cited herein. PAOs are included as the subject of numerous textbooks, such as Lubrication Fundamentals, J. G. Wills, Marcel Dekker Inc., (New York, 1980), and Synthetic Lubricants and High-Performance Functional Fluids, 2nd Ed., Rudnick and Shubkin, Marcel Dekker Inc., (New York, 1999).
A major trend in passenger car engine oil usage is the extension of oil drain intervals. Due to tighter engine oil performance, a need exists for low viscosity PAO products with improved physical properties, e.g., evaporation loss as measured by, for instance, Noack volatility, as well as excellent cold weather performance, as measured by, for instance, pour point or Cold Crank Simulator (CCS) test. Noack volatilities are typically determined according to ASTM D5800; pour points are typically determined according to ASTM D97; and CCS is obtained by ASTM D5293.
The properties of a particular grade of PAO are greatly dependent on the α-olefin used to make that product. In general, the higher the carbon number of the α-olefin, the lower the Noack volatility and the higher the pour point of the product. PAO's having a nominal viscosity at 100° C. of 4 cSt are typically made from 1-decene and have a Noack volatility of 13-14% and pour point of <−60° C. PAO's having a nominal viscosity at 100° C. of 6 cSt are typically prepared from 1-decene or a blend of α-olefins and have a Noack volatility of about 7.0% and pour point of about −57° C.
U.S. Pat. No. 5,789,355 relates to SAE Grade 5W and higher multigrade oils including a basestock and a detergent inhibitor package. The basestock is selected from API Groups I and II (or Groups 1 and 2, respectively). The detergent inhibitor package includes an ashless dispersant derived from an ethylene alphaolefin (EAO).
In U.S. Pat No. 6,646,174, a mixture of about 10 to 40 wt. % 1-decene and about 60 to 90 wt. % 1-dodecene and are co-oligomerized in the presence of an alcohol promoter. Preferably 1-decene is added portion-wise during the single oligomerization reactor containing 1-dodecene and a pressurized atmosphere of boron trifluoride. Product is taken overhead and the various cuts are hydrogenated to give the PAO characterized by a kinematic viscosity of from about 4 to about 6 at 100° C., a Noack weight loss of from about 4% to about 9%, a viscosity index of from about 130 to about 145, and a pour point in the range of from about −60° C. to about −50° C.
An improvement on the conventional processes is disclosed in U.S. Pat. No. 6,824,671. A mixture of about 50 to 80 wt. % 1-decene and about 20 to 50 wt. % 1-dodecene are co-oligomerized in two continuous stirred-tank reactors in series using BF3 with an ethanol:ethyl acetate promoter. Monomers and dimers are taken overhead and the bottoms product is hydrogenated to saturate the trimers and higher oligomers to create a 5 cSt PAO. This product is further distilled and the distillation cuts blended to produce a 4 cSt PAO containing mostly trimers and tetramers, and a 6 cSt PAO containing trimers, tetramers, and pentamers. The lubricants thus obtained are characterized by a Noack volatility of about 4% to 12 %, and a pour point of about −40° C. to −65° C. See also U.S. application Ser. No. 10/959544.
U.S. Patent Application 2004/0033908 is directed to fully formulated lubricants comprising PAOs prepared from mixed olefin feed exhibiting superior Noack volatility at low pour points. The PAOs are prepared by a process using an BF3 catalyst in conjunction with a dual promoter comprising alcohol and alkyl acetate, and the products are the result of blending of cuts.
In general, the lower the molecular weight of the alcohol used in the synthesis, the lower the degree of polymerization and the lower the viscosity of the product. Temperature, residence time, pressure and the concentration of the catalysts also affect the degree of polymerization. However, the normal alcohol used is the major factor that affects the degree of polymerization. Conventional processes not only produce trimers but also tetramers and higher oligomers. Unfortunately, large quantities of trimers, which are in great demand, are not produced by prior art methods even with the use of low molecular weight normal alcohol. Moreover, the separation of the trimers made by prior art methods, usually by distillation, produces other PAOs that are not in great demand.
A PAO that is lighter than 4 cSt at 100° C. can also be made, by way of example, blending PAOs that have nominal 100° C. viscosity of about 2 cSt and about 4 cSt, respectively, such as disclosed in U.S. Pat. No. 6,703,353 and U.S. patent application Ser. No. 2004/0094453. However, such a products have relatively high Noack volatility.
A product having a combination of low viscosity, low Noack volatility, and good cold temperature properties is highly desirable. The present inventors have surprisingly discovered a method of producing oligomers, including a high proportion of trimers, characterizable by low viscosity, low Noack volatility, and excellent low temperature fluidity, without the necessity of blending.